As a LED (light emitting diode), particularly as a blue or an UV LED that has an emission wavelength of approximately 250 nm to 550 nm, a high-brightness LED product using a compound semiconductor of a GaN compound such as GaN, GaAlN, InGaN or InAlGaN has been obtained. Also, a high-definition full color image may be formed using a technique to combine red and green LEDs with a blue LED. For example, a technique to prepare a white LED by combining a blue LED or an UV LED with a phosphor has been known. Such LEDs have been increasingly required for general lighting or backlight in liquid crystal displays (LCDs).
As an encapsulant for the LEDs, an epoxy resin that has high adhesive strength and excellent dynamic durability has been widely used. However, the epoxy resin has problems in that it has low transmittance with respect to light having blue to UV wavelength regions, and also shows deteriorated lightfastness. Therefore, techniques of solving the above-described problems have been proposed, for example, in Japanese Patent Laid-Open Publication Nos. Hei11-274571, 2001-196151 and 2002-226551. However, encapsulants described in this literature show insufficient lightfastness.
As a material having excellent lightfastness with respect to a low wavelength range, a silicone resin has been known. However, the silicone resin has disadvantages in that its heat resistance is poor, and its surface is sticky after curing. Also, in order to effectively apply the silicone resin as an LED encapsulant, characteristics such as a high refractive property, crack resistance, surface hardness, adhesive strength and thermal shock resistance must be secured.